Spacecraft such as satellites orbiting the Earth to view or monitor the Earth require to be aligned and maintained in alignment with the Earth. Movement away from a nominal earth pointing attitude requires to be sensed for a correction to be made to return the satellite to the nominal Earth pointing direction. Conventionally the attitude of an Earth orbiting spacecraft or satellite relative to the Earth is determined by a sensor assembly which is sensitive to illumination in the infra-red waveband, i.e. in the range of from 12 to 14 microns. Such a conventional Earth attitude determining sensor operates by repeatedly scanning the field of view of a small number, typically, four sensors sensitive to infra-red radiation across the Earth/space horizon. The roll and/or pitch attitude error of the spacecraft or satellite relative to the nominal Earth pointing direction is obtained from timing information from these horizon crossings. Such a conventional Earth attitude determining sensor is prone to reliability problems. For example scanning of the sensors is carried out mechanically by means of movable scanning mirrors and these scanning mirrors can stick due to heat problems from radiation from the sun or from the other parts of the spacecraft or satellite. Additionally such a conventional attitude determining assembly is also subject to interference from other celestial bodies such as the Moon or Sun and from reflections from other parts of the spacecraft or satellite which can be confused with an Earth/Space crossing.